As is generally known, a loudspeaker driver is a device that converts electrical energy into acoustic energy or sound waves. In its simplest form, a typical loudspeaker driver consists of a coil of wire bonded to a cone or diaphragm and suspended such that the coil is in a magnetic field and such that the coil and cone or diaphragm can move or vibrate perpendicular to the magnetic field. An electrical audio signal is applied to the coil and the suspended components vibrate proportionally and generate sound.
Although cone and horn-type speakers are very common, other types of loudspeakers, such as planar magnetic loudspeakers are also well-used. A planar magnetic loudspeaker is a type of ribbon that has a lightweight, flat diaphragm suspended in a frame between magnets of alternating polarity. When current passes through the conductive traces that are bonded to the diaphragm, the traces move backward or forward in the magnetic field, causing the diaphragm to move. The term “planar” refers to the magnetic field that is distributed in the same plane (parallel) to the diaphragm. Planar magnetic diaphragms are thin and lightweight as opposed to the much heavier moving-coil or dome diaphragms found in “dynamic” drivers. The diaphragm is suspended in the magnetic fields created by the magnetic arrays and a printed circuit spread across the surface of a thin-film substrate is energized with an audio signal to interact with the magnetic field and produce an electromagnetic force that moves the diaphragm back and forth to create sound waves.
FIG. 1A illustrates a planar magnetic loudspeaker 103 comprising a diaphragm frame 102 holding diaphragm 104 upon which are bonded conductive traces 108. Magnets 106 set up a magnetic field that creates the force to move the diaphragm in response to audio signal current passing through the conductive traces. A case having an upper case portion (or half) 101a and a lower case portion 101b surrounds and holds the diaphragm 102 and includes a plurality of openings or ports 110 through which the sound wave from moving diaphragm 104 is projected.
FIG. 1B illustrates the example diaphragm and the arrangement of the conductive traces for the planar magnetic loudspeaker of FIG. 1A. As shown in FIG. 1B, the conductive traces are laid out and bonded onto diaphragm 104 in an appropriate coil configuration to distribute the electric signal over the area of the diaphragm within frame 102. Signal wires 112 coupled to the conductive traces provide the audio signal from an amplifier or audio playback system to the loudspeaker 103.
FIG. 1C illustrates an example assembled planar magnetic loudspeaker driver for the diaphragm of FIG. 1B. As shown in FIG. 1C, diaphragm 104 is placed between the upper and lower case portions 101a and 101b. The upper case portion 101a has openings 110 arranged to allow the sound projected sound waves to pass out from the moving diaphragm. The number, size, and arrangement of the openings 110 may be of any appropriate configuration depending on the size, shape, material, and power rating of the loudspeaker, along with other relevant characteristics.
Physical surfaces such as horns or waveguides are commonly used to control the sound dispersion of planar magnetic drivers. FIG. 1D illustrates an example planar magnetic loudspeaker driver with waveguides 112, which are added to the front of the driver to control the horizontal dispersion angle of sound waves from the diaphragm or ribbon transducer 104. The surfaces shown are approximately 45 degrees either side of the direction of sound, relative to the vertical axis. As such they limit the horizontal sound dispersion angle or beamwidth to approximately 90 degrees. FIG. 1D also illustrates certain angle notations relative to the driver axes. As shown, the vertical axis 114 is assumed to be the long axis of the planar magnetic loudspeaker driver, and the horizontal axis 116 is assumed to be the short axis of the driver. The nominal direction of sound projection (in monopole operation) 118 is out the front of the driver at 0 degrees vertical and 0 degrees horizontal, as shown in FIG. 1D.
FIG. 1E illustrates an example measured dispersion pattern for the loudspeaker and waveguide arrangement in FIG. 1D. For this example, the exit height is 120 mm and the exit width, between the waveguides, is 24 mm. The horizontal beamwidth holds at approximately 90 degrees between approximately 5 kHz and 14 kHz. As can be seen in plot 120, above 14 kHz the beamwidth narrows as the sound wavelength becomes smaller than the width of the exit. FIG. 1F shows the measured vertical dispersion pattern for the loudspeaker arrangement in FIG. 1D. As can be seen in plot 130, above approximately 2.8 kHz the beam narrows as the sound wavelength becomes smaller than the height of the exit. At high frequencies, the vertical beamwidth is only a few degrees and only a listener positioned directly on axis to the loudspeaker will hear all frequencies at a similar sound level. This plot thus shows a disadvantage associated with present planar magnetic loudspeakers with regards to limited sound dispersion, namely narrow dispersion and relatively high directivity. Many applications require a loudspeaker to cover an audience area larger than just a few degrees either side of the aiming direction and as such, the planar magnetic loudspeaker driver is unsuitable.
What is needed therefore, is a planar loudspeaker system or manifold that improves dispersion of sound from the driver, and especially increases the vertical beamwidth of the loudspeaker.
The subject matter discussed in the background section should not be assumed to be prior art merely as a result of its mention in the background section. Similarly, a problem mentioned in the background section or associated with the subject matter of the background section should not be assumed to have been previously recognized in the prior art. The subject matter in the background section merely represents different approaches, which in and of themselves may also be inventions.